Effect of iron saturation on the bacteriostasis of human serum: in vivo does not correlate with in vitro saturation.

Human serum inhibits the growth of a variety of human pathogens. One of the serum bacteriostatic components is transferrin, the major iron-binding protein. In the presence of transferrin, free iron, which is required for bacterial nucleoprotein synthesis, is unavailable and bacterial growth is inhibited. In an in vitro system, we tested the hypothesis that serum with highly saturated transferrin allows free iron to be available for rapid bacterial growth. We first confirmed the finding that addition of ionic iron sufficient to saturate transferrin in normal sera inhibits the bacteriostatic activity for Escherichia coli. In contrast, no differences were found in the growth rate of E. coli in sera from individuals representing the entire range of transferrin saturation found in humans (iron-deficient, normal, and thalassemic). This finding supports the thesis that iron added in vitro is more easily extracted than in vivo, where it is tightly bound to transferrin, and does not support the contention that ordinary iron treatment predisposes infants to infection.